NMR Parameter Measurements

A novel MAS NMR approach has been presented for the determination of motionally averaged heteronuclear dipolar couplings in unoriented materials. The technique is based on the proton-detected local field protocol and achieves dipolar recoupling by R-type RF irradiation. The experiment, which is called R-PDLF spectroscopy, is demonstrated on solid and liquid-crystalline systems. For mobile systems, it is shown that the R-PDLF scheme provides better dipolar resolution as compared to techniques combining conventional separated local field spectroscopy with R-type recoupling. 

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Fig. 1 Solid-state NMR structure analysis relies on the 19F-labelled peptides being uniformly embedded in a macroscopically oriented membrane sample, (a) The angle (0) of the 19F-labelled group (e.g. a CF3-moiety) on the peptide backbone (shown here as a cylinder) relative to the static magnetic field is directly reflected in the NMR parameter measured (e.g. DD, see Fig. 2c). (b) The value of the experimental NMR parameter varies along the peptide sequence with a periodicity that is characteristic for distinct peptide conformations, (c) From such wave plot the alignment of the peptide with respect to the lipid bilayer normal (n) can then be evaluated in terms of its tilt angle (x) and azimuthal rotation (p). Whole-body wobbling can be described by an order parameter, S rtlo. (d) The combined data from several individual 19F-labelled peptide analogues thus yields a 3D structural model of the peptide and how it is oriented in the lipid bilayer...

Little difference was found between the NMR results on the LP and HP samples at either room temperature or as the temperature was raised, although the T1 for the HP sample was 10% shorter than that for the LP sample. After maintaining the samples at temperatures near 200°C for 1 hr., a major difference was noted upon cooling, with the HP sample exhibiting a marked time-temperature hysteresis of all of the measured NMR properties while the LP sample exhibited no time-temperature hysteresis in any of the NMR parameters measured. A more dramatic manifestation of this is that after the sample was cooled to room temperature, the proton motion took weeks to return to the original state... 

To conclude, this study [124] shows the strong potential of combining EFG DFT calculations and accurate quadrupolar NMR parameter measurements for the structural and electronic characterizations of crystalhne inorganic materials. 

Another principal difficulty is that the precise effect of local dynamics on the NOE intensity cannot be determined from the data. The dynamic correction factor [85] describes the ratio of the effects of distance and angular fluctuations. Theoretical studies based on NOE intensities extracted from molecular dynamics trajectories [86,87] are helpful to understand the detailed relationship between NMR parameters and local dynamics and may lead to structure-dependent corrections. In an implicit way, an estimate of the dynamic correction factor has been used in an ensemble relaxation matrix refinement by including order parameters for proton-proton vectors derived from molecular dynamics calculations [72]. One remaining challenge is to incorporate data describing the local dynamics of the molecule directly into the refinement, in such a way that an order parameter calculated from the calculated ensemble is similar to the measured order parameter. 

Table 1 Interactions in solid state NMR, parameters, their selective measurement, and...

The results of NMR measurements have to be converted into a 3D structure. After establishing the constitution by NMR parameters that are transmitted through bond, i.e. J-coupHng constants, information about the spatial structure is introduced. Here, mainly distances from NOE build-up rates are used to define the configuration and conformation. 

The number of NMR parameters available for measurement is rather small, consisting of the chemical shift, relaxation rates (/1 and lo), scalar (J) couplings, dipolar (D) couplings, cross-relaxation rates (the NOE), and hydrogen exchange rates. All of these have been quantified for many of the amide protons of A131 A, and most of the data suggest the presence of little persistent structure. 

The 9Be NMR spectroscopy has been used in determination of the chiral binaphthyldiimine Be(II) complex [49].118 The measured 8Be value of 4.84 ppm and the line width of 40 Hz indicated the presence of a four-coordinate complex. The calculated 9Be NMR parameters were in good agreement with experimental results. 

Several local labels need to be measured, usually one-by-one in individual samples in the case of 19F-NMR. The combined set of anisotropic NMR parameters then allows one to re-construct the geometry of the entire peptide and to determine its alignment in the membrane, as illustrated in Fig. 1 [35-37, 47, 48]. The only prerequisite is that the 19F-labelled moiety has to be rigidly attached to the peptide backbone, and that the peptide assumes a well-defined secondary structure. Provided that a sufficient number of local orientational constraints can be measured... 

Despite the benefits of high resolution, measurements of wideline spectra of quadrupolar nuclei under static or MAS conditions are still commonly used in a variety of applications. For both integer and half-integer spins, simulations of quadrupolar lineshapes can yield full sets of NMR parameters associated with quadrupolar and chemical shift tensors and can be used for studying molecular dynamics. 

Section 3, the main section of this paper, deals with the NMR of bulk semiconductors. Section 3.1 lists the various relevant terms in the NMR spin Hamiltonian. The NMR techniques and strategies that can be employed to obtain the individual NMR parameters of the spin Hamiltonian and theoretical calculations of NMR parameters will be discussed in Sect. 3.2. The remaining subsections will provide examples from the important classes of semiconductors that illustrate the measurement and interpretation of each of the spin Hamiltonian parameters, with an emphasis on what information about semiconductors the parameters convey. 

The isotropic chemical shift, the trace of the chemical shift tensor, is one of the basic NMR parameters often measured for both spin-1/2 and quadrupolar nuclei. The CSA can also be measured in non-cubic environments, such as the n3Cd nuclei experience in the chalcopyrite structure of crystalline CdGeAs2 [141] or CdGeP2 [142], and the 31P nuclei in the latter compound [142], Although the isotropic chemical shift can be measured from the NMR spectrum of a static powder because the CSA is zero in many cases because of cubic symmetry of the lattice, improved resolution is obtained by using MAS to remove dipolar couplings. Two particular areas where the isotropic chemical shifts have proven very informative will now be discussed, semiconductor alloys and semiconductor polytypes. 

Most of the methods are based upon the application of both a static magnetic field gradient and a mechanical wave in order to measure the consequence of the latter on a given NMR parameter. Indeed, the mechanically-induced... 

In principle, there are different ways to characterize the complexes On one hand, in favourable cases, it is possible to measure the NMR parameters of the receptor in the free and bound states, i.e., differences in chemical shift and NOE-contacts, before and after the formation of the complex. On the other hand, it is usually more feasible to observe and detect changes in the ligand NMR signals before and after the binding process. In this latter case, the use of STD (saturation transfer difference) and TR-NOE (transferred NOE) experiments is of paramount importance. 

The following descriptions will be couched primarily in terms of chemical shift changes, which are the most widely used for the measurement of dissociation constants the general principles apply equally to measurements based on other NMR parameters, and these will be touched on later. 

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